Dengue virus (DV) is a single-stranded RNA Flavivirus that causes dengue fever (DF), the most prevalent arthropod-borne viral illness in humans. Dengue viruses cause an estimated 25 million new cases of DF and 250,000 cases of dengue hemorrhagic fever (DHF) per year in tropical and subtropical areas of the world, with >100 countries with endemic transmission. Currently, no specific treatment or licensed vaccine is available for either DF or DHF. Given its global burden, increased travel and military activity n dengue-endemic areas, there is an urgent need for safe therapeutics. This is a 4-year grant application to test three genetically modified neutralizing monoclonal antibodies (mAbs), for efficacy in treating the clinical signs of DF or DHF in humanized mice, caused by all four DV (1-4) serotypes. The humanized mouse model of dengue developed by the Rico-Hesse laboratory is the only one that shows consistent signs of disease as in humans, after infection with low-passage, or clinical isolates of the virus, via mosquito bite. We have streamlined the production of these mice, using NOD/SCID/IL2gamma null mice and human hematopoietic stem cells from birth cord blood and we also now infect these mice by mosquito bite, to mimic natural transmission. We expect to validate that these mAbs, which are modified in their Fc regions to eliminate the possibility of antibody-dependent enhancement of infection (ADE), are safe and effective therapeutic agents for infection by all serotypes of dengue virus. We will test independently and together in humanized mice a strongly neutralizing anti-envelope protein antibody (E60) that recognizes the highly cross-reactive fusion loop on domain II and a second antibody (either 1A1D-2 or 4E11) that binds the conserved A-strand of domain III and is serocomplex- specific (i.e., all dengue serotypes). Humanized and modified forms of these mAbs will be tested as a post-infection therapeutic in the humanized mouse model (days 3 and 7 post-infection). We also propose to test for ADE in vivo, after passive transfer of two mAb cocktails and infection with one DV2 virus, to compare parent and modified mAbs, and whether they cause severe dengue clinical signs in humanized mice. The production, testing, and validation of these modified humanized antibodies in a unique and more relevant animal model of disease will foster the development of novel immunotherapeutics against DV, an NIAID category A pathogen.